You Asked!Q. I've heard you should replace running shoes every 300 to 500 miles. But what if I wear mine on the elliptical or stationary bike - how should I tally those miles?A. 'Since those activities don't wear out your shoes as quickly as running does, you should only count 25 to 50 percent of those miles. The impact of your foot hitting the ground breaks down the cushiony middle layer in your sneaker... That's why your joints might feel achy after running in a worn-our pair.'